Fir 1 Epithermal Neutron Beam Model and Dose Calculation for Treatment Planning in Neutron Capture Therapy

The epithermal neutron beam model of the Finnish boron neutron capture therapy (BNCT) facility (FiR 1) was created using the two-dimensional (2D) discrete ordinates transport (DORT) code. The final design of the beam was achieved using the DORT model: the optimal thickness of the neutron moderator and the length and the thickness of the bismuth collimator of the beam were calculated. The final beam model was validated experimentally with dosimetric measurements. The computed neutron beam spectrum was first verified with activation measurements free in air. Suitable brain tissue substitutes for neutron capture therapy (NCT) dosimetry were examined. The computed thermal neutron fluence [and gold (Au) and manganese (Mn) activation reaction rates], the gamma dose and the fast neutron dose distributions in the three tissue substitute (TS) phantoms were verified with activation and pair ionisation chamber measurements. The simplified neutron-photon beam model for the treatment planning system (TPS) was determined from the DORT model. The TPS beam model was experimentally validated in the three TS phantoms. The beam model was normalised to the Au activation measurements at the thermal neutron maximum in the PMMA (polymethylmethacrylate) phantom, which gave a link to the monitor units. The planned radiation dose in the TPS is given in monitor units. The experimentally verified beam model was first applied in the computations of the dose plans of the dog brain and in the treatment planning of glioblastoma multiforme (GBM) patients in the Finnish BNCT project. The 2D cylinder symmetrical horizontal DORT model of the FiR 1 epithermal neutron beam was observed to be an effective and reliable tool for examining the effects of different geometrical structures (moderator, collimator) on neutron and photon spectra. Of the simple phantom materials, PMMA was found to simulate the thermal neutron fluence at its maximum in the brain tissue 3 percentage units closer than water in the collimated epithermal neutron beam. However, water simulated the absorbed gamma dose in the brain tissue 12 percentage units closer than PMMA. In addition, a brain tissue equivalent liquid was designed. Parallel verification of the beam model in water, PMMA and the brain equivalent liquid confirmed reliability of the NCT dose computation. The DORT beam model was sufficiently accurate (intensity correction 5%) to use as a beam model in TPS. The beam model was normalised at the thermal neutron maximum in the PMMA phantom with the Au activation measurements. The use of the calculated Au activation reaction rate (variation 3%) for the normalisation was found to be less independent of the energy grouping of cross sections than the calculated Mn activation reaction rate (variation 13%). The soft tissue, bone and air cavities need to be defined separately to create an accurate threedimensional (3D) head model of the target area for the BNCT treatment planning. The accuracy of the beam model can be roughly estimated with in vivo dosimetry, which is recommended for use at epithermal neutron facilities in accordance with new protocols. The